In the current cellular systems, a "cell" comprises an omnidirectional antenna or directional antennae which each usually covers a sector of the cell, radio frequency receivers and transmitters dedicated in frequency, converters, multiplexers and switching and control equipment to provide first level cell processing, supervision and handoff as well as the supervisory communications to a mobile telephone switching office (MTSO).
Each MTSO contains a substantial amount of equipment and is connected to the cell equipment of the cells within its area of supervision and to a telephone network or Public Switched Telephone Network (PSTN) by land lines, e.g. coaxial cable or conventional telephone lines. Generally speaking, the MTSO includes a switching machine which provides the connection between the trunks or lines from the PSTN and the trunks or lines from the MTSO to the cells and a control system for managing the radio frequency (RF) receiver channels and the RF transmitter channels active at each cell and the interconnection of the trunks from the PSTN and such channels. The MTSO is more complicated than a conventional central telephone office because of the additional level of complexity in call supervision and call routing. This additional level of complexity is required because the network path for individual calls can change several times during a single telephone conversation. Such changes can involve one or any combination of:
a. The radio frequency which the call is using; PA1 b. The sector antenna at a cell site which is handling the call; PA1 c. The cell site which is handling the call; and PA1 d. The transmitted radio frequency power for each channel at the cell site and station. PA1 a. Constant monitoring in order to optimize the cell's capacity and the utilization of the spectrum as traffic grows and calling patterns change; PA1 b. Continuous re-deployment and re-arrangement of equipment at the cell sites; and PA1 c. Corresponding re-configuration and adjustments to the capacity of the communications links between the cell and the MTSO.
A main drawback of the current systems is that because of the frequency restrictions of the land lines interconnecting the cell equipment with the MTSO equipment, the cell equipment must convert the data modulated radio frequency energy which it receives to a form which can be transmitted by the land lines and it must convert the data which it receives from the MTSO, which is restricted in frequency by the land lines, to data modulated radio frequency energy to be transmitted. As a result, the cell must include not only the radio frequency receiver and transmitters and modulators but also equipment for:
Consequently, the equipment at the cell site is expensive and increases maintenance and deployment of maintenance spare equipment at the site.
The amount of radio frequency and processing equipment at a site defines the site's wireless service capacity and capabilities. The individual radio frequency receivers and transmitters at a site are assigned specific frequencies that define the number of available active channel links at the site. The operating frequencies of the receivers and transmitters are assigned in accordance with specific wireless protocols, which divide the cellular spectrum into blocks of channels to avoid the well known problems of co-channel and adjacent channel interference among cell sites in a cellular system. The switching and processing equipment at each site manage data routing, e.g., pulse width modulated, voice frequency or frequency shift key data, for active channel links established between an antenna and the MTSO, supervise call initiation and call hand-off between antennae and monitor received overhead data, such as, for example, polling data which identifies cellular receivers/transmitters within the detection range of the site.
The MTSO also includes processing equipment for managing active channel links between cell sites and mobile or fixed receiver/transmitters, such as a cellular phone or pager. The equipment further controls and manages routing of overhead and communication data transmitted on active channel links within the entire system.
The fragmentation of call surveillance, monitoring and control functions among the cell sites and the MTSO in a cellular system results in the following disadvantages. First, substantial expense must be incurred for changing cell site capacity and protocol service features for radio frequency equipment in a cell site to permit that different or larger portions of the radio frequency spectrum may be used for establishing active channel links at the cell site. In addition, the high cost of RF equipment imposes practical limitations on the number of RF units which will be included at a cell site. Also, the addition of new RF equipment at a cell site may require expensive re-configuration or re-deployment of land line connections between the cell site and the MTSO to accommodate increased capacity at that specific site. Further, it is very difficult to change active channel link connections from one antenna to another within a cell site or between antennae of different cell sites, or to change the RF transmission power, receiving range or frequency of transmission for an active channel link at a cell site in response to changes in wireless traffic and calling patterns in the geographic coverage area of the cellular system.
Therefore, the current cellular systems have many disadvantages.